Highly reflecting anodised al surfaces with tailored diffuse and specular content

ABSTRACT

The present invention relates to a method to obtain a reflective anodized aluminium surface on an object. The present invention relates in particular to a method to obtain a reflective, anodized aluminium surface having a white appearance.

FIELD OF THE INVENTION

The present invention relates to a method to obtain a reflectiveanodized aluminium surface on an object. The present invention relatesin particular to a method to obtain a reflective, anodized aluminiumsurface having a white appearance.

BACKGROUND OF THE INVENTION

Whereas most colours can be produced by absorption, this is not the casefor white, since it is a combination of all visible wavelengths oflight. White appearance of aluminium cannot be seen analogous to thecoloured appearance of dyed aluminium, as it conceptually is verydifferent.

White surfaces are ubiquitous in a huge number of applications (windowframes, panels, doors, lamps, etc.), and while a white surface can beachieved using paint or even white plastics, a white wear-resistantaluminium surface would often be the number one choice if such a surfacewould be available.

White aluminium surfaces can be produced by embedding titanium dioxide(TiO₂) or other white pigments into an anodic film. The white pigmentsopacify the films primarily by diffusely reflecting light. Thisreflection occurs because the white pigment scatters or bends lightstrongly. If there is enough white pigment in an anodic film almost allvisible light striking it will be reflected, and the anodic film willappear opaque, white, and bright.

The scratch and UV-resistance of an anodized surface is significantlyhigher than that of a traditional painted surface. Hence, anodizedsurfaces are usually preferred to painted surfaces when it comes to bothpractical applications and long-lasting decorative purposes. Thereforewhite anodized surfaces are preferred and have high value compared tothe white painted aluminium.

Embedding white pigments into an anodic film is not a straight-forwardoperation, considering that the pigments are typically magnitudes largerthan the nanoscaled pores that are created in an anodizing process. Itis known from EP 2 649 224 B1 to obtain a radiation scattering surfacefinish on an object by providing the object with a top layer, comprisingaluminium or an aluminium alloy, the top layer comprising added discreteinclusions of a second material being different from aluminium and thefirst alloy, and subsequently anodizing said top layer to form an anodicoxide layer and to generate from the inclusions discrete radiationscattering elements. In an embodiment said radiation scattering elementsare selected from particles of titanium, tin, zirconium, iron, titaniumoxide, tin oxide, zirconium oxide, and iron oxide. The process describedwill secure that pigments, such as white pigments, are embedded into ananodic alumina film, which together will provide the scatteringmechanism that finally generates a surface that is being perceived aswhite.

An anodizing method using high-frequency switching anodizing (HSA) isdisclosed in “Anodizing method for aluminum alloy by usinghigh-frequency switching electrolysis” H. Tanaka, M. Fujita, T.Yamamoto, H. Muramatsu Suzuki Motor Corporation; H. Asoh, S. Ono,Kogakuin University.

Multi-pass friction stir processing (FSP) to impregnate TiO₂ particlesinto the surface of an aluminium alloy and subsequent anodizing in asulphuric acid electrolyte is disclosed by V. C. Gudla, F. Jensen, A.Simar, R. Shabadi, R. Ambat: Friction stir processed Al—TiO₂ surfacecomposites: Anodizing behaviour and optical appearance, Appl. Surf. Sci.324 (2015) 554-562.

High frequency anodizing of friction stir processed Al—TiO2 surfacecomposites using a high frequency pulse and pulse reverse pulsetechnique at a fixed frequency in a sulfuric acid bath is disclosed byV. C. Gudla, F. Jensen, K. Bordo, A. Simar, R. Ambat, Effect of HighFrequency Pulsing on the Interfacial Structure of AnodizedAluminium-TiO2, Journal of The Electrochemical Society, 162 (7)C303-C310 (2015).

Multi-pass friction stir processing (FSP) to impregnate metal oxide(TiO₂, Y₂O₃ and CeO₂) particles into the surface of an aluminium alloyand subsequent anodizing in a sulphuric acid electrolyte is disclosed byV. C. Gudla, F. Jensen, S. Canulescu, A. Simar, R. Ambat, Friction stirprocessed Al—metal oxide surface composites: anodization and opticalappearance, 28th international conference on surface modificationtechnologies, Jun. 16-18, 2014, Tampere University of Technology,Tampere, Finland.

US 2009/0236228 A1 relates to an anodizing method and apparatus.

US 2006/0037866 relates to an anodic oxide film and anodizing method.

US 2008/0087551 relates to a method for anodizing aluminum alloy andpower supply for anodizing aluminum alloy.

JP2004-035930 relates to an aluminum alloy material and anodizationtreatment method therefore.

JP2008-0085574 relates to a method for anodizing an aluminum member.

JP2007-154301 relates to an aluminum alloy anodic oxidation method andpower source for aluminum alloy anodic oxidation.

Existing technologies for creating a white anodic film are mainlylimited to matt surfaces. Furthermore, existing technologies rely on afundamentally different light scattering mechanism provided by astructured surface that scatters light diffusely and that does notprovide the same degree of whiteness as obtained through the presentinvention.

Thus, there is a need for developing a process technology capable ofprocessing aluminium generating visually appealing, wear-resistant,white alumina surfaces.

OBJECT OF THE INVENTION

It is an object of embodiments of the invention to provide beautiful,wear-resistant anodized aluminium (Al) surfaces with novel opticalappearances through a white anodized alumina surface layer (Al₂O₃).Furthermore, the appearance of the surfaces can be tuned the entire wayfrom matt-etched white to a very bright glossy white.

SUMMARY OF THE INVENTION

It has been found by the present inventor(s) that by providing an objectwith a top layer comprising aluminium or an aluminium alloy, the toplayer comprising embedded discrete particles of a metal or metal oxide,said metal being different from aluminium, and anodizing said top layerin an aqueous solution of an organic acid applying a time varyingsignal, such as a high frequency signal of square wave pulses, adecoratively appealing, wear-resistant white alumina surface isobtained.

So, in a first aspect the present invention relates to a method toobtain a reflective anodized aluminium surface on an object, comprisingthe steps:

-   -   a. Providing the object with a top layer comprising aluminium or        an aluminium alloy, the top layer comprising embedded discrete        particles of a metal or metal oxide, said metal being different        from aluminium;    -   b. Subsequently anodizing said top layer to form an anodic oxide        layer;        wherein said anodizing of step b. takes place in an aqueous        solution of an organic acid applying a time varying signal.

LEGENDS TO THE FIGURE

FIG. 1(a) shows almost parallel growth of DC formed anodic pores, makingit hard to reach the regions underneath particles;

FIG. 1(b) shows branched pores formed during high frequency anodizing;and

FIG. 2 illustrates the process disclosed in example 1.

DETAILED DISCLOSURE OF THE INVENTION Specific Embodiments of theInvention

In an embodiment of the invention the embedded discrete particles areselected from the group consisting of titanium, tin, zirconium, iron,titanium oxide, tin oxide, zirconium oxide, lead oxide, yttrium oxide,and iron oxide, preferably titanium oxide.

Titanium dioxide (TiO2) exhibits a light refractive index much differentthan sealed anodic alumina, making it an ideal pigment for obtaininggood light scattering. As such, pigments with other chemicalcompositions can be used, if they possess properties similar to those oftitanium dioxide.

In an embodiment of the invention the particle size of the embeddeddiscrete particles is in the range 100-500 nm, preferably in the range150-400 nm, such as 200-300 nm. The size of TiO₂ particles shouldpreferably be 200-300 nm to secure light scattering of all visiblewavelengths, making the surface perceived as white.

In an embodiment of the invention the aluminium or aluminium alloycomprises at least 95% by weight of aluminium, preferably at least 96%by weight of aluminium, such as at least 97% by weight of aluminium,such as at least 98% by weight of aluminium, more preferably at least99% by weight of aluminium.

A pure alumininum alloy is required for the anodic film to become asoptically transparent as possible. Alloying elements such as Fe, Mn andCu must be kept to an absolute minimum, knowing that these elements willgive rise to a certain degree of light absorption, which will compromisethe anodic film whiteness. Using an alloy with a composition equivalentto a 6060 (or even purer), has proven to give good results.

In an embodiment of the invention the discrete particles of a metal ormetal oxide are embedded by a solid state process.

Non-limiting examples of solid state processes include a solid stateprocess selected from the group consisting of friction stir processing(FSP), additive friction stir processing (AFSP), and powder metallurgy.

Friction stir processing (FSP) is a solid state process known for itsability to modify microstructures and provide improved properties overconventional processing technologies. The development of friction stirprocessing (FSP) is based on the friction stir welding (FSW) technology.FSW works by plunging a spinning tool into the joint of two materialsand then traversing the rotating tool along the interface. The frictioncaused by the tool heats up the materials around the pin to atemperature below the melting point. The rotation of the tool “stirs”the material together and results in a mixture of the two materials. InFSP a specially designed rotating pin is first inserted into thematerial to be processed with a proper tool tilt angle and then movedalong the programmed paths. The pin produces frictional and plasticdeformation heating within the processing zone. As the tool pin moves,materials are forced to flow around the pin. Material flows to the backof the pin, where it is extruded and forged behind the tool. It isevident that FSW and FSP share the same mechanism, however, havecompletely different purposes in practical applications. The goal of FSWis to join two plates together, whereas FSP aims at modifying themicrostructure of a single or multiple workpieces.

Additionally, FSP has emerged as an advanced tool to produce surfacecomposites by embedding second phase particles into the matrix. It isexactly this feature that is utilized in this patent application as toembed white pigments into the aluminium bulk, considering that the FSPprocess has the required advantages of:

i) Maintaining a sufficiently low temperature to avoid a criticalreaction between pigment and aluminium;

ii) Being able to remove excess heat via a Heat Sink, again to avoid areaction between pigment and aluminium;

iii) Securing a homogenous and individual distribution of pigmentswithin the aluminium matrix; and

iv) Leaving the pigments in a functional state.

Whereas Friction Stir Processing (FSP) is a very time-consuming batchprocess, Additive Friction Stir Processing (AFSP) has emerged to createa continuous process, where particles are fed to the material through ahollow spinning tool. Not only is this a much faster (and non-batch)process, it also allows for much higher particle loadings.

Even though AFSP is the preferred technique for embedding white pigmentsinto an aluminium matrix, other techniques are also available. Anfurther example of solid state processing (route 2) is powdermetallurgy, where pigments are mechanically alloyed into the aluminiumpowder. The composite powder is subsequently compressed and shaped in anormal powder metal route such as forging, cold isostatic pressing(CIP), hot isostatic pressing (HIP), direct profile extrusion, directrolling of sheets, cold spraying, thermal spraying etc.

In another embodiment of the invention the discrete particles of a metalor metal oxide are embedded by a liquid state process, such as e.g. StirCasting or Investment Casting.

In another embodiment of the invention the discrete particles of a metalor metal oxide are embedded by a vapour state process such as e.g.Physical Vapour Deposition (PVD) or Chemical Vapour Deposition (CVD).

As such, any of the above major processing routes can be used, as longas they fulfill the considerations mentioned above. Anodizing securesthe conversion of aluminium into aluminium oxide. The pigments whichwere embedded into the top aluminium layer will become embedded into thealuminium oxide after anodizing.

The difference in refractive index between the anodic oxide and thewhite pigments secures scattering of all visible wavelengths thatfinally makes the anodized surface appear white.

Anodic films are traditionally formed by passing a direct current (DC)through an electrolyte, with the aluminium part working as the anode anda suitable material serving as the cathode. However, DC anodizing hasproven problematic in anodizing the aforementioned composite alloy, dueto the regions that are underneath each individual pigment. Anodic poresformed through a DC process are almost completely parallel and do notreach the regions underneath the pigments. This leaves an anodic filmwith embedded pigments that have a small area of non-anodized aluminiumunderneath them. In turn this is a very unfortunate situation,considering the light absorption properties of metallic aluminium, whichfinally makes the entire anodic film be perceived as dark rather thanwhite.

High Frequency Anodizing has proven much more advantageous compared totraditional DC anodizing, due to the branching nature of the pores,which extend all the way underneath each individual pigment. This willsecure an anodic film entirely depleted of non-anodized aluminium, asshown in FIG. 1.

Like traditional hard anodizing the white anodizing must be carried outat low temperature and in low-aggressive electrolyte to decrease thedegree of pore wall attack.

The electrolyte used in an anodizing process is traditionally waterbased and has an active content of acid. Almost all weak and strongorganic acids can function as an electrolyte in the anodizing step ofthe process according to the invention.

In an embodiment of the invention the anodizing of step b. takes placein an aqueous solution of an organic acid selected from the groupconsisting of oxalic acid, succinic acid, tartaric acid, malic acid,maleic acid, formic acid, citric acid and acetic acid. In a preferredembodiment of the invention the anodizing of step b. takes place in anaqueous solution of an organic acid selected from the group consistingof oxalic acid, formic acid and citric acid, preferably oxalic acid.

The high frequency signal, which is a time varying signal, may comprisea square wave signal having pulses with amplitudes between between −5 Vand +5 V in the low period and between +15 V and 100 V in the highperiod. Moreover, the voltage ramp up/down times of the pulses may be inthe range between 0 and 15% of the ideal square wave pulse duration. Thefrequency of the square wave signal may typically be around 1 kHz.

The thickness of the anodized film determines how white the surfaceappears. To secure a total white light scattering effect in the visiblespectrum—normally about 100 μm oxide is necessary. Thus in an embodimentof the invention the thickness of the anodized film is in the range50-300 μm, such as about 75-200 μm, preferably in the range 100-150 μm,such as in the range 80-130 μm.

The pigment concentration determines how white the surface appears. Thusin an embodiment of the invention the pigment concentration is in therange 2-25 wt %, such as about 5-20 wt %, preferably in the range 10-15wt %.

The anodizing parameters will determine the quality of the anodizedfilm. Thus. optical properties can be characterized by a standardspectrophotometer, where the degree of reflected light is measured.

Hardness can be measured with a standard microhardness testing unit,where a diamond indenter is pressed into the surface. The diagonal (incase of Vickers hardness testing) of the resulting indentation gives afigure for the surface hardness.

Tribological properties can be found by a standardized wear tester suchas a ball-on-disc setup.

The thick anodic film obtained above may be slightly dissolved in theupper part because of the prolonged exposure to the acid electrolyte, aphenomenon known as “pore wall attack”.

The porous oxide can be stabilized by impregnating it with an agent thatfills the anodic pores.

Thus, in an embodiment of the method according to the invention themethod comprises a further step of impregnating the anodized aluminiumoxide layer.

In an embodiment of the invention said impregnation is performed bymeans of an impregnating substance selected from the group consisting ofa silicate, a lacquer, and a sol-gel substance. Non-limiting examples oflacquers and sol-gel substances include acrylics, silanes and silanebased sol-gels.

EXAMPLE 1

Aluminium plates with dimensions 200 mm×60 mm×6 mm were used for the FSPtrials. Commercial TiO₂ powder in rutile phase was used. The mediandiameter of the powder particles was 210 nm. Processing the FSP processwas performed using a hermle milling machine equipped with a steel toolhaving 20 mm shoulder diameter, 1.5 mm pin length with a m6 thread. Thebackwards tilt angle of the tool was maintained at 1°. A groove 0.5 mmdeep, 10 mm wide, and 180 mm long in the Al plates which was compactlyfilled with TiO₂ powder. The filled plates were then covered by the sameAl sheet rolled down to a thickness of 0.25 mm to prevent loss of TiO₂powder during the initial FSP pass. Rotational speed of the tool was1000 rpm and the advancing speed was 200 mm/min for the first pass toinsure correct closure of the groove and 1000 mm/min for the next sixpasses. A surface of 175 mm long×20 mm wide was processed for each passwith a total processing time of roughly 2 min. All seven passes wereperformed one over the other without any shift. The samples were thenmechanically polished, buffed to a mirror finish and then degreased in amild alkaline solution at 60° C. The samples were subsequently desmuttedby immersing in diluted HNO₃ followed by demineralized water rinsing.Anodizing was carried out in a saturated oxalic acid bath maintained at10° C. A square wave high frequency signal of 1 kHz from 0 to 40V wasapplied, with a controlled ramp up/down duration which corresponds to10% of the pulse duration. The process continues until the filmthickness has grown to approximately 100 μm. After anodizing the surfaceappears white, with both specular and diffuse reflections. The sample isrinsed and transferred to a hot water sealing tank for closing theopen-pored anodic structure. The process is illustrated in FIG. 2.

1.-15. (canceled)
 16. A method to obtain a reflective anodized aluminiumsurface on an object, comprising the steps: a. Providing the object witha top layer comprising aluminium or an aluminium alloy, the top layercomprising embedded discrete particles of a titanium or titanium oxide,b. Subsequently anodizing said top layer to form an anodic oxide layer;wherein said anodizing of step b. takes place in an aqueous solution ofan organic acid applying a time varying signal.
 17. The method accordingto claim 16, wherein the particle size of the embedded discreteparticles is in the range 100-500 nm, preferably in the range 150-400nm, such as 200-300 nm.
 18. The method according to claim 16, whereinthe aluminium or aluminium alloy comprises at least 95% by weight ofaluminium, preferably at least 96% by weight of aluminium, such as atleast 97% by weight of aluminium, such as at least 98% by weight ofaluminium, more preferably at least 99% by weight of aluminium.
 19. Themethod according to claim 16, wherein the discrete particles of atitanium or titanium oxide are embedded by a solid state process. 20.The method according to claim 19, wherein said solid state process is aprocess selected from the group consisting of friction stir processing(FSP), additive friction stir processing (AFSP), and powder metallurgy.21. The method according to claim 16, wherein the discrete particles ofa titanium or titanium oxide are embedded by a liquid state process. 22.The method according to claim 16, wherein the discrete particles of atitanium or titanium oxide are embedded by a vapour state process. 23.The method according to claim 16, wherein the anodizing of step b. takesplace in an aqueous solution of an organic acid selected from the groupconsisting of oxalic acid, succinic acid, tartaric acid, malic acid,maleic acid, formic acid, citric acid and acetic acid.
 24. The methodaccording to claim 23, wherein the organic acid is selected from thegroup consisting of oxalic acid, succinic acid, tartaric acid, malicacid, maleic acid, and citric acid.
 25. The method according to claim24, wherein the time varying signal comprises a high frequency signal inthe form of a square wave signal having a frequency between 500 Hz and 5kHz, such as around 1 kHz.
 26. The method according to claim 25, whereinthe square wave signal has an amplitude between −5 V and 100 V, such asbetween 0 V and 40 V.
 27. The method according to claim 25, wherein thesquare wave signal comprises ramp up and/or ramp down times between 0and 15% of a pulse duration.
 28. The method according to claim 16,further comprising a further step of impregnating the anodized aluminiumoxide layer.
 29. The method according to claim 28, wherein saidimpregnation is performed by means of an impregnating substance selectedfrom the group consisting of a silicate, a lacquer, and a sol-gelsubstance.